This application claims priority from G. Maris, Italian Patent Application No. TO2000A000278, filed Mar. 23, 2000, the disclosure of which is incorporated by reference herein in its entirety.
The present invention relates to a mixing element for screws of extruders with two co-rotating screws and to an extruder comprising the mixing element.
The outer profile of a conventional mixing element can be produced (FIG. 1) as a result of the removal of material from a circular element 1 along at least one arc of its outer portion, by cutting performed by a point-like element 2 rotating around a circle 3 with a diameter equal to that of the circular element 1. By way of indication, the point-like cutting element 2 may, for example, be considered as the trace in the plane of the drawing of a laser beam perpendicular thereto. During the cutting, the point-like element 2 and the circular element 1 rotate in the same direction and at the same speed and the centers of the circular element 1 and of the circle 3 are spaced apart by a constant distance less than their diameter.
The curves indicated by the reference numerals 4 in FIG. 1 represent the positions adopted successively by the circular element being cut as a result of successive rotations through 100 about its center, and the ends of the radii 5 of the circle 3 represent successive points of contact of the point-like element 2 with the circular element 1.
For reasons of practicality of implementation, the actual method of producing known mixing elements may differ from the theoretical method just described. The final result, however, is the same, that is, the removal of a certain portion of the circular element 1; for example, the removal of a crescent 9 as a result of a rotation through 90xc2x0, which results in the production of the element 6a shown in FIG. 2a. 
By performing the cutting operation starting from several different points, it is possible to produce mixing elements with different profiles, for example, those distinguished by the references 6b, 6c and 6d, shown in FIGS. 2b, 2c and 2d, respectively.
According to the prior art, a plurality of mixing elements 6a-6d are keyed onto the shafts of the two screws of an extruder by means of respective central toothed holes 7 and the mixing elements of each screw are intended to work in cooperation with respective homologous elements mounted facing them on the shaft of the other screw.
The screws rotate in the same direction and at the same speed. The portions of the outer profiles of the homologous mixing elements facing one another at any particular time on the straight line joining the centers of these elements therefore define a space of constant width, through which the polymeric material being processed passes and is thus subjected to a mechanical stress.
The width of the space depends both on the spacing with which the two screws are positioned and on the reduction in the extent of the mixing element, in comparison with the circular element 1 from which it has been formed.
In determining the width of this space, the need to increase it in order to avoid imparting excessively high shear forces to the material is tempered by the need to reduce it in order to confer on the mixing element a width such that it can absorb the mechanical torque transmitted to it by the motor driving the respective screw, even in the narrowest portion (see reference numeral 8 in FIG. 2b).
In the end, a compromise value, which cannot be completely satisfactory with regard to one or both of the abovementioned requirements, is thus selected.
The object of the present invention is to provide a mixing element for extruders with two screws by virtue of which the shear force exerted on the material being processed in the extruder is reduced without thereby comprising the ability of the mixing element to absorb the torque transmitted to it.
According to the invention, this object is achieved by means of a mixing element, as well as an extruder comprising the mixing element and a processing method performed thereby, having the specific characteristics recited in the appended claims.